Triphyllozoon microstigmatum Silén, 1954

Martino, Emanuela Di, 2023, Scanning electron microscopy study of Lars Silén’s cheilostome bryozoan type specimens in the historical collections of natural history museums in Sweden, Zootaxa 5379 (1), pp. 1-106 : 74-76

Triphyllozoon microstigmatum Silén, 1954

( Fig. 40 ; Table 35)

Triphyllozoon microstigmatum Silén, 1954: 26 , fig. 11, pl. 2, fig. 8.

Material examined. Holotype by monotypy LUZM 53 , off north of Cottesloe , Western Australia; depth 11–22 m. Leg. Prof. T. Gislén, Australia Expedition 1951–1952, collected 19.2.1952.

Description. Colony erect, rigid, reticulate; holotype specimen 5 cm high and 2.2 × 1.2 cm in diameter ( Fig. 40A ), arched and tubular with the autozooidal openings on the inside; fenestrae oval ( Fig. 40C, J ), 0.65–1.00 × 0.40–0.60 mm, and trabeculae consisting of 2–7 alternating autozooidal series, more commonly 3–5 ( Fig. 40C, H ).

Autozooids rectangular to hexagonal, longer than wide (mean L/ W 1.60), distinct with boundaries marked by raised margins of smooth calcification ( Fig. 40C–E ); frontal shield tubercular, flat to slightly convex, imperforate except for a few sparse, round, elliptical or slit-like pores along the lateral and proximal margins, 12–40 µm in diameter/length ( Fig. 40C–E, H ).

Peristome deep, forming a raised collar around the orifice and a shallow (15–25 µm), narrow (10–15 µm), median U-shaped pseudosinus proximally ( Fig. 40D, F ) that extends as a channel ( Fig. 40I ) towards the operculum on one side of a short and square median process ( Fig. 40E, H ); peristomial avicularium and oral spines absent; secondary orifice subcircular, cormidial, produced by two or three autozooids.

A medium-sized, adventitious, frontal avicularium on almost every zooid (exceptionally two), mainly two types recognizable: type (1) round avicularia (mean L/ W 1.02) with raised, finely denticulate rostrum and semicircular mandible ( Fig. 40E , see white arrows); type (2) parallel-sided elliptical (mean L/ W 1.52) and flat avicularia with semielliptical mandible ( Fig. 40E , see black arrows). Both types of frontal avicularia placed either proximally, in line with the zooidal axis and directed proximally or proximolaterally, or placed laterally at about zooidal mid-length and directed laterally; both types with mandibles as long as the rostra. Rarely, a type (3) larger avicularium, 185 µm long by 80 µm wide, placed on the frontal of zooids adjacent to the fenestrae, with bicuspid rostrum directed distally, and triangular mandible exceeding the length of the rostrum (mandible length 180 µm; rostrum length 125 µm) hooked at the tip ( Fig. 40F, G ). On the dorsal side two types of avicularia: type (4) medium-sized pear-shaped avicularia (mean L/ W 1.73) randomly directed with semielliptical mandible ( Fig. 40L, M ), and type (5) circular avicularia (mean L/ W 1.03) protruding from the edge of fenestrae ( Fig. 40K ). All avicularia with complete crossbar.

Ovicell hyperstomial, prominent; ooecium slightly wider than long (mean L/ W 0.81), tubercular as the frontal shield, with dentate suture highly variable in shape from trifoliate and Y-shaped to bifoliate shaped as a horizontally flipped L ( Fig. 40I ) or with the single lateral lobe down curved ( Fig. 40H ); median suture 70–90 µm long. Labellum short and square.

Dorsal side coarsely granular (granules diameter 10–20 µm), with vertical and oblique vibices (10–15 µm wide), outlining irregularly polygonal sectors, and usually two avicularia as described above per sector at the two opposite corners, commonly adjacent to or protruding from edge of the fenestrae ( Fig. 40J–M ).

Remarks. Silén (1954) identified the dorsal, circular avicularia protruding from the edges of numerous fenestrae ( Fig. 40J , see arrows) as hydroid tube openings. He attributed these structures to Zanclea protecta based on their similarity with those made by this hydroid in other species of Triphyllozoon ( Harmer 1934) . Although, it is wellknown that the bryozoan skeletal material offers protection to these hydroids (e.g. McKinney 2009; Hirose & Hirose 2012), SEM observations of crossbars ( Fig. 40K ) prove that these structures are authentic avicularia.

Figures

FIGURE 40. Triphyllozoon microstigmatum Silén, 1954. Holotype LUZM 53, Western Australia.A. View of the tubular colony. B. Labels accompanying the specimen. C. Group of autozooids. D, E. Close up of autozooids with frontal avicularia, some with open or closed mandibles. Arrows indicate the two varieties, round with raised, finely denticulate rostrum (white arrows), and elliptical flat (black arrows). F. Large avicularium with bicuspid rostrum located at the fenestra. G. Close-up of the avicularium in (F) showing the bicuspid rostrum and the triangular, hooked mandible. H. Group of zooids, two of which ovicellate. I. Close-up of an ooecium. J. General view of the dorsal side. Arrows point to circular avicularia protruding from the edge of some fenestrae. K. Close-up of circular avicularia protruding from the dorsal edge of the fenestrae. L, M. Close-ups of dorsal avicularia. Given the tubular shape of the colony exposing the dorsal side of the autozooids towards the exterior and the frontal side towards the interior, SEM micrographs were taken from two fragments subsampled from the tips of the holotype colony. Scale bars: A = 2 cm; C, F, H = 500 µm; D = 400 µm; E = 300 µm; G, I = 100 µm; J = 1 mm; K = 45 µm; L, M = 50 µm.